Preparation
1. Preheat oven to 375 degrees. Spray a muffin pan with cooking spray.
2. Heat the oil in a medium skillet over medium heat.
3. Add the onion and asparagus and cook, stirring occasionally until the vegetables have softened, about 6 minutes.
4. Add the spinach and cook until wilted. Allow to cool slightly.
5. Meanwhile, in a medium bowl, whisk the eggs and milk together.
6. Stir in the Parmesan cheese, the cooked vegetables, salt and pepper.
7. Distribute the mixture evenly into the muffin pan.
8. Bake until set in the center, 18-20 minutes.
9. Serve warm or at room temperature.